The builder should add many additives to improve the pigment’s dispersion, the product’s storage stability, the coating’s appearance, the construction performance, and the like. These surfactants change the coating’s surface tension to make it easy to produce foam during the construction process, especially the water-based coating. The danger of foam: (A) Limiting productive capacity; (B) Affecting product quality; (C) Causing waste; (D) Polluting the environment; (E) Accidents may occur. Therefore, to eliminate these adverse effects, the defoamer becomes an indispensable auxiliary agent in aqueous coatings.
(A) When mixing and dispersing in the production process, and air is introduced during the construction process, bubbles are generated due to the change of the free energy of the system.
(B) The addition of wetting dispersing agent, emulsifier, and other additives can reduce the surface tension, which helps the generation and stabilization of foam.
(C) The use of thickening agent thickens the bubble wall, which makes it challenging to eliminate the bubbles.
2. Factors Affecting Foam Stability
The factors affecting foam stability are:
(1) The viscosity of the solution
(2) Surface tension of the solution
(3) Surface viscosity of the foam
(4) The structure of surfactant
(5) Marangoni effect
(6) The elasticity of the foam
(7) Other influencing factors
The other influencing factors include temperature, solvent volatilization rate, pH value, surfactant adsorption speed, foam size, foam impact degree, the interaction of various components in the system, and the like.
The main factors that affect the stability of the foam are different in different systems, and multiple factors exist simultaneously and interact collectively. The elasticity of the foam is the main factor affecting the stability of the foam.
Foam is an unstable system that will spontaneously change from a state with higher free energy to a state with lower free energy, obeying the laws of thermodynamics.
The defoaming mechanism comprises:
(A) To destroy the elasticity of the membrane
(B) To promote liquid film drainage
(C) The addition of defoamers reduces the foam’s local surface tension and causes the foam to break.
(D) The addition of low-molecular defoamer ethanol, propanol, and the like reduces the surface layer’s surfactant concentration and dissolves into the surfactant adsorption layer to destroy the foam stability.
The composition of the defoamer is shown in the following Table A.
Table A: The composition and functions of the defoamer are as below. | ||
COMPOSITION | FUNCTION | CHEMICAL COMPOSITION |
ACTIVE SUBSTANCE | TO BREAK AIR BUBBLES AND DEFOAM | SILICONE OIL, POLYETHERS, HIGHER ALCOHOLS |
CARRIER | TO HELP THE SYSTEM AND ACTIVE SUBSTANCES TO COMBINE AND DISPERSE | AROMATIC HYDROCARBONS, ALIPHATIC HYDROCARBONS, OXYGENATED SOLVENTS |
EMULSIFIER | TO HELP ACTIVE SUBSTANCES TO DIFFUSE AND SPREAD | NONYL (OCTYL) PHENOL, SOAP SALT, POLYOXYETHYLENE ETHER, TWEEN SERIES, OP SERIES, SPAN SERIES |
EMULSIFICATION ADDITIVE | TO DISPERSE AND INCREASE VISCOSITY | HYDROPHOBIC SIO2, POLYVINYL ALCOHOL |
The defoamer enters the liquid film spontaneously and spreads and disperses rapidly on the interface. The defoamer changes the interfacial properties of the liquid film, thus causing the film to break.
There are also differences in the way different types of defoamers destroy bubbles. For example:
(A) Tributyl phosphate reduces the liquid film’s surface viscosity so that the liquid film drainage speed increases to achieve the defoaming effect.
(B) Alcohol and ethers spread on the interface, and the surface tension decreases wherever they go. By pulling the liquid in the foam to the high surface tension, alcohol and ethers render the film becomes thin, and it ruptures.
The types, characteristics, and scope of application of defoamers suitable for water-based coatings are shown in the following Table B.
Table B: The type, characteristics, and scope of application of defoamer in the water-based coating. | ||
TYPES OF DEFOAMERS | PERFORMANCE | SCOPE OF APPLICATION |
METALLIC SOAPS, MINERAL OIL WAX | MINERAL OIL AND WAX SOAP HAVE POOR COMPATIBILITY WITH THE SYSTEM, AND FLOATING AFFECTS TRANSPARENCY | THE BUILDER USES THIS TYPE OF DEFOAMER IN INDUSTRIAL PRIMER OF HIGH POLYVINYL CHLORIDE (PVC) SYSTEM, BUT NOT IN HIGH GLOSS SYSTEM. |
SILICONE | STRONG DEFOAMING, HIGH FOAM-INHIBITABILITY, AND GOOD THERMAL STABILITY | THIS TYPE OF DEFOAMER HAS A WIDE RANGE OF APPLICATIONS. BUT, THE BUILDER CANNOT USE THIS TYPE OF DEFOAMER IN A SYSTEM WITH STRONG ACIDITY AND ALKALINITY BECAUSE THIS TYPE OF DEFOAMER IS PRONE TO DEMULSIFICATION. |
POLYETHER BLOCK | GOOD COMPATIBILITY, LITTLE INFLUENCE ON GLOSS, AND WEAK FOAM INHIBITION PERFORMANCE | THE BUILDER USES THIS TYPE OF DEFOAMER IN VARNISH AND HIGH GLOSS SYSTEM. |
BLENDED | GOOD COMPATIBILITY AND LESS COATING DEFECTS | THIS TYPE OF DEFOAMER HAS A WIDE APPLICATION RANGE. THE BUILDER CANNOT APPLY THIS TYPE OF DEFOAMER TO TOPCOAT, AND THE LIKE. |
For a builder, the principle of defoamer selection is based on the following aspects.
(1) Strong defoaming ability
(2) Good stability
(3) There is no obstacle to recoating.
(4) It does not affect other aspects of performance.
Finally, the builder should determine the defoamer selection through experiments according to the resin type, formula system, construction method, production process, and the use of other additives.
Table C: The dosage of defoamer is shown as below. | |||
SYSTEM | LOW-VISCOSITY LATEX PAINT AND WATER-BASED PAINT | HIGH-VISCOSITY LATEX PAINT | OTHER WATER-BASED PAINTS |
W(DEFOAMER)/% | 0.01 ~ 0.2 | 0.3 ~ 1.0 | AROUND 0.1 |
The defoamer is usually added when the viscosity is high. A portion of the defoamer is added in the grinding and dispersion stage. Then, another portion of the defoamer is added to the mixing stage.
In the application, the challenges that the constructor should pay attention to are as follows.
(A) In different production processes, the usage of defoamers is also different. When the construction method is different, the dosage of the defoamer is also different. When applying paint with rolling, air will be brought in, and bubbles will increase. As a result, the builder should add more defoamer appropriately.
(B) The compatibility issues: If the defoamer’s compatibility with the system is inadequate, turbidity, floating, and even color changes will occur, and the adhesion between layers will also be affected.
(C) Stir evenly before using defoamer. Because defoamer is a mixture of several different properties and functions, it is easy to be delaminated.
(D) The influence of temperature: In the resin synthesis stage, a high-temperature defoamer should be used. Otherwise, the defoamer will demulsify and lose the defoaming effect.
(E) The influence of pH value: In a strong acid and strong base system, if a silicone defoamer is used, serious demulsification will occur.
MIKEM defoamer is a powder defoamer, belonging to a modified polyether. When applied in coating, it provides the following features:
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